In recent years, hybrid vehicles (HV), plug-in hybrid vehicles (PHV) and electric vehicles (EV) are finding wider use. Secondary batteries (cells) or rechargeable batteries are installed in these environment-friendly vehicles as key devices. As an in-vehicle secondary battery, a nickel hydride battery (cell) and a lithium ion battery (cell) are mainly in use. Since the lithium-ion has a high energy density and has no memory effect, it is anticipated that the spread of the lithium-ion is accelerated in years to come.
In order to achieve high output and large capacity, the in-vehicle secondary battery is used such that a plurality of unit cells are connected adjacently to each other. Normally, the in-vehicle secondary battery is provided as a battery pack where a plurality of battery blocks are coupled together. Here, in each of the plurality of battery blocks, a plurality of unit cells are arranged.